Stainless steels

ABSTRACT

Disclosed are high-strength stainless steels of the following composition: ElementWeight Percent Carbon 0.12 to 0.20Nitrogen 0 to 0.07Chromium12 to 16Molybdenum 0 to 7Vanadium 0 to 0.6Cobalt12 to 16Nickel 0.5 to 1.5Niobium 0.1 to 0.3Iron Balance These steels exhibit high strength, fracture toughness, ductility and uniform elongation.

O United States Patent 11 1 1111 3,873,378 Webster Mar. 25, 1975 [5 1STAINLESS STEELS 3,340,048 9/1967 Floreen 148/38 [751 Invent 3 Webster,Island, ijififili if 02824711111111:31113113132111: 132/33 [73]Assignee: The Boeing Comppany, Seattle, Primary Lovell w h. Attorney,Agent, or Firm-Christensen, OConnor,

G & H 1k 22 Filed: Dec. 13, 1972 ave a [21] Appl. No.: 314,686 [57]ABSTRACT Related Application Data Piselosed are highstrength stainlesssteels of the fol- [62] Division of S61. No. 171,181, Aug. 12, 1971,Pat. No. Owmg compo Element Weight Percent Carbon 0.12 to 0.20 [52] US.Cl 148/37, 148/125, 148/135 g 13 [51] Int. C22C 39/10, C22C 41/04, C2ld1/18 Molybtlenum 0 to 7 [58] Field of Search 148/37, 38, 125, 134, 135,Xgggfl g 148/143 Nickel 0.5 1015 Niobium 0.1 to 0.3 [56] ReferencesCited Baum UNITED STATES PATENTS These steels exhibit high stength,fracture toughness, 3,131,097 4/1964 Mantel 148/143 X ductility anduniform elongation 3,154,412 10/1964 Kasak 148/37 3,251,683 5/1966Hammond 148/37 8 Claims, 4 Drawing Flgures resistance and decreasedfatigue crack growth rate. It is another object of this invention toprovide heat treatment procedures by which the mechanical properties ofthe steels of this invention can be optimized.

SUMMARY OF THE INVENTION STAINLESS STEELS This is a division, ofapplication Ser. No. 171,181, Brow? Range 3 Preferred a ge filed Aug.12, 1971 now U.S. Pat. No. 3,756,808. (Wlght%) wei ht 7%) Carbon 0.12 to0.20 0.13 to 0.18 BACKGROUND OF THE INVENTION 5 Nitrogeln to 0.07 0.02 m005 This invention relates to stainless steels and more 1 2:; i8 2:?particularly to hardenable stainless steels which exhibit Vanadium 0 ,to0.6 0.1 to 0.4 Cobalt 12 to 16 13.2 to 14.2 combinations of highstrength, toughness, ductility and Nickel to L5 085 to I L25 uniformelongation not heretofore obtainable. Niobium to ()3 on to 0,25

Two recently introduced stainless steels, AFC-77 l0 Balance Balance andAFC-260 (both developed by Crucible Steel Company of America See p NO26,225) Preferably, the elements are individually maintained sesscombinations of strength and elongation which are the Preferred rangesmdlcatedlmpurlties Such superior to previous stainless steels. Thecompositions as slllcofli manganese, f Phosphorus can be of these twoSteels are Shown in Table 77 is present in the steels of this invention.Silicon and manmartensitic in character, while AFC-260 issemiaustegallese Contents Should generally be maintained below time 10.35 and 0.25 weight percent, respectively. Sulfur and The compositionsof both are balanced such that heat phosphorus contents should each bemaintained below treatment will substantially eliminate retainedaustenabo 0 5 weight p and p eferably are mainite. AFC-77 can beheat-treated to strengths of up to 20 tamed below about 0.010 weightpercent. about 290 ksi by tempering at from 900 to 1,100 F. Typical ofthe new steels of this invention is a steel However, its fracturetoughness and ductility are limerred to herein as Alloy B which has thefollowing ited by the small amounts of austenite retained at suchCOH'IPOSIIIOIII 7 TABLE I AFC-77 (wt. 7 AFC-260 (wt. 76 Element NormalRange Aim Normal Range Aim Carbon 0.13 to 0.17 0.15 0.05 to 0.09 0.07Silicon 0 to 0.30 0.20 to 0.35 0.25 Manganese 0 to 0.25 0.15 to 0.300.25 Sulfur 0 to 0.010 0 to 0.015 Phosphorus 0 to 0.010 0 to 0.015Nickel 0 to 0.25 1.7 to 2.0 1.85 Chromium 14.0 to 14.7 14.5 15.2 to 15.815.5 Molybdc- 4.7 to 5.1 5.0 4.25 to 4.75 4.5 num Vanadium 0.3 to 0.40.5 None Cobalt 13.0 to 14.0 13.5 12.6 to 13.6 13.0 Niobium None 0.10 to0.25 0.15 Nitrogen 0.03 to 0.06 0.05 0.01 to 0.05 0.03 Iron BalanceBalance Balance Balance tempering temperatures. In its austeniticcondition, ement Weight% AFC-260 is characterized by low yield strengthand Carbon 0J6 reasonable ductility. With thermal treatment of AF-gliqn'oge n 51,332 C-260 to effect austenite-to-martensitetransformammlum I M 1 bd 5.22 tion, tensile strengths of about 265 ksican be obtained. g i jg 009 Up to its maximum tensile strength, AFC-260does exgol leg; hibit good toughness. }f, It is an ob ect of thisinvention to provide high Silicon 0.06 strength stainless steelsexhibiting combinations of 2133 32 83( strength, toughness, ductilityand uniform elongation Phosphorus 0.018 which are superior to those ofAFC-77, AFC-260 and Balance other prior art steels. It is a furtherobject of this inven- I tion to provide high strength stainless steelsof the type Thi invention i l di d to a novel h described which alsoexhibit improved stress corrosion treatment process b hi h th i r ha ial properties of the steels of this invention can be optimized.

BRIEF DESCRIPTION OF THE DRAWINGS Various mechanical properties of thesteels of this invention and comparisons thereof with prior art steelsare graphically depicted by the accompanying drawings, wherein:

FIG. 1 shows tensile stress-strain curves for AFC-77 and AFC-260 andtensile stress-strain curves for Alloy B showing the effect of variousheat treatments on strain to fracture values:

FIG. 2 shows a comparison of the toughness and tensile strength of AlloyB with existing stainless steels; and

FIG. 3 shows a comparison of elongation values of Alloy B with existingstainless steels.

FIG. 4 shows a comparison of the uniform elongation values of Alloy Bwith those of existing stainless steels.

DETAILED DESCRIPTION OF THE INVENTION The compositions of the steels ofthis invention are critically balanced such that, even after completeheat treatment, they have a duplex structure consisting of a dispersionof on the order of from to of soft austenite in a matrix of hardmartensite. The retained austenite, however, is sufficiently unstablethat it transforms to martensite when stressed, the enhanced strengthand ductility of these steels being attributed to the occurence of thistransformation preferentially at regions of highest stress. Duringtensile tests of Alloy B, areas which attempt to neck raise the localstress at that portion of the gage length and cause the formation ofstress-induced martensite. This hardens the local area sufficiently toobviate any further tendency to neck. This process occurringcontinuously along the entire gage length insures a high uniformelongation.

Composition-wise, the steels of this invention can be considered to bemodifications of AFC-77, the modifications being'the inclusion of 0.5 to1.5% by weight of nickel and 0.1 to 0.3% by weight of niobium. Theniobium addition effects refinement of the austenite grain size whichincreases strength and stress corrosion resistance and decreases fatiguecrack growth rate. The nickel addition stabilizes the austenite, causingmore austenite to be retained at high tempering temperatures.

AFC-260 can also be considered a modification of AFC-77 (see Table I),the most significant differences between the two being that AFC-260contains 1.7 to 2.0% by weight nickel and 0.10 to 0.25% by weightniobium and has a reduced carbon content (0.05 to 0.09%). The lowercarbon content in AFC-260 results in lower strength and tends to negatethe austenitestabilizing effect of the nickel.

The preferred procedure for heat treating the steels of this inventionis as follows:

In Stage I, the steel is austenitized at from l,600 to l,800 F., andpreferably at about I,700 F., and then cooled to ambient temperature.This stage is designed to refine the austenite grain size of the steelby optimiz ing the size and dispersion of the niobium carbides.Austenitizing for only a few seconds at the indicated temperatures willeffect some austenite grain refinement, but times of at least one hourare preferred. The rate of cooling to ambient temperature is notcritical and can be in air or by oil quenching.

Stage 2 involves austenitizing at from 1,950 to 2,300 F. for at leastabout one-quarter hour, cooling directly to a lower temperature of from1,800 to 2,000 F and holding within the latter range for at least aboutone-half hour to remove delta ferrite, an undesirable brittle phase. Thesecond temperature should be at least 50 F. lower than the first. Themethod and rate of cooling are not critical. Preferably this stage iscarried out by austenitizing at from 2,000 to 2,200 F. for at least onehour, cooling to from 1,850 to 1,950

F. (preferably about l,900 F.) and holding for at least about I hourwithin the latter range. Holding for one hour at about 1,900 F. issufficient in most cases to remove all delta ferrite but holding timesof as much as hours can be beneficial if alloy segregation is severeenough to markedly slow the removal rate of delta ferrite. After theholding period, the steel is cooled in air or by oil quenching. Thesteel can be further cooled to -65 F. to -l50 F. (preferably to aboutl00 F.) and held there for from one-half hour to 20 hours to effectfurther hardening by transforming austenite to martensite. The subzerocooling step can be omitted to reduce yield strength and increaseelongation and toughness.

Stage 3 involves tempering the steel at temperatures of from 500 toI,l00 F. for at least one-half hour and preferably for 2 2 hours (2hours at temperature followed by air cooling to ambient temperaturefollowed by an additional 2 hours at the same temperature). A variationon this tempering treatment is to temper at two different temperatures,the first being a low one designed to render the austenite more stableat the second higher tempering temperature. A promising range for thefirst tempering temperature has been found to be from 650 to 750 F.(preferably about 700 F.). Some increase in elongation can be achievedfor a given final tempering temperature by first tempering at 700 F. for2 hours. However, this increased ductility is obtained with somesacrifice in strength.

The heat-treatment procedure described above has general applicabilityto stainless steels comprising from 0.0] to 0.25% by weight carbon, IIto 16% by weight chromium, l0 to 20% by weight cobalt and up to 10%molybdenum.

One of the most important properties of the steels of this invention ishigh uniform elongation, i.e., the elongation before local necking orreductions in area occur. In FIG. 1 are shown the tensile stress-straindiagrams for AFC-77 (FIG. 1A), AFC-260 (FIG. 1B) and Alloy B in fivedifferent heat-treated conditions (FIG. 1C 1G). The AFC-77 and AFC-260specimens tested had been subjected to the following heat treatments(see U.S. Pat. No. 3,563,813): austenitizing at 2,IO0 F., cooling to andholding at l,900 F. for 1 hour, oil quenching to ambient temperature andtempering for 2 2 hours at 900 F. (AFC-77) and l,000 F. (AF- C-260). TheAlloy B specimens used to produce the five heat-treated conditions hadbeen austenized at l,700 F. for 1 hour, cooled to room temperature,austenized at 2,lO0 F cooled to and held for one hour at 1,900 F. andthen cooled to room temperature by oil quenching. To produce Condition I(FIG. 1C), Alloy B was tempered at 800 F. for 2 2 hours without havingbeen previously cooled to subzero temperatures. In this condition AlloyB contains substantial amounts of retained austenite which produces alow yield point (-40 ksi), but its work hardening capacity is sufficientto produce an ultimate tensile strength of 238 ksi. Alloy B in thiscondition is particularly applicable whenever extensive plasticdeformation in a fully heat-treated condition is required. In conditionII (FIG. 1D), Alloy B is tempered at 800 F. for 2 2 hours after asubzero treatment (1 hour at l00 F.) which removes a large amount ofretained austenite. This results in a substantial increase in yieldstrength with only a slight decrease in ductility. The effect of coldworking on Alloy B in Condition II has been examined by cold rolling0.080 inch sheet. Cold reductions up to 68% were obtained withoutedge-cracking. There is a significant increase in both yield and tensilestrengths as a result of cold working, and tensile strengths of up to384 ksi have been obtained by this technique. Ageing at 800 F. after theworking produces a further increment of strength. The elongation dropsrapidly with cold working but ductility as measured by reduction of arearemains at a high level for a material of this strength.

In Conditions III, IV and V (FIG. 1E IG), Alloy B was subjected to asubzero treatment as in Condition II and then was tempered at 900 F. for2 2 hours (Condition Ill); tempered at 700 F. for 2 hours, cooled toroom temperature and tempered at 800 F. for 2 2 hours (Condition IV); ortempered at 700 F. for 2 hours. cooled to room temperature and temperedat 900 F. for 2 2 hours (Condition V). Alloy B in Condition III has ahigher strength than in Condition II, at

strength steels. Uniform elongation is a measure of a materialsformability and is also a design parameter in some bending applications.

The corrosion resistance, strength, toughness and ductility of thesteels of this invention render them useful in many diverseapplications. One application for the steels of this invention is theproduction of highstrength rivets which are tough and ductilevenough tobe gun driven without cracking, and which, after driving, have shearstrength of over 170 ksi. The rivet must possess its high ductility in afully hardened condition, since further heat treatment in situ is notusually practical. The properties of Alloy B in Conditions I and II arecompared in Table II with a widely used stainless rivet material A 286.

* Measured by a double shear test on V. inch diameter bar. Testedaccording to the requirements of Aerospace Research and TestingCommittee report number ARTCflflfl-aslener Ensron. Double Shear and LapJoint Testing Procedures."

some sacrifice in toughness and elongation. As dis- From Table II itwill be observed that Alloy B offers a cussed previously, the 700 F.treatment in Conditions IV and V, stabilizes the austenite and preventsits transformation to martensite at the higher temperatures (800 900 F).This results in some increase in both total and uniform elongation, witha slight decrease in strength.

One of the objects of this invention is to provide a stainless steelpossessing a combination of strength and toughness not available incommercial steels. The extent to which this object has been achieved canbe seen from FIG. 2 which compares Alloy B in Conditions II, III and VIwith existing stainless steels. Condition VI was produced in the samemanner as Conditions II V except that tempering was at l,000 F. for 2 2hours. The fracture toughness of Alloy B in Condition I is so high thatmeasurement thereof was impractical because of the large size ofspecimen that would have been required. The steepness of the fracturetoughnessstrength relationship for AFC-77 is a reflection of thediminishing austenite content at the higher strength levels. In AFC-260the austenite is maintained at a high level up to the maximum strengthof 265 ksi so that the strength-toughness line is considerably lesssteep than for AFC-77. Alloy B retains austenite up to an ultimatestrength of 290 ksi and exhibits a better combination of strength andtoughness than either AFC77 or AF- considerably higher tensile and shearstrengths while maintaining a high level of toughness. Duringinstallation of the rivets by either gun driving or squeezing, Alloy Bin Condition 1 work hardens rapidly due to martensite formation so thatthere is a marked increase in shear strength from 126 ksi to I74 ksi.Condition I material is the most suitable for gun driving, since its lowyield strength will allow formation of the bucktail with less force thanwould normally be required for a material of this shear strength.

Another application for the steels of this invention is the manufactureof razor blades. The following is a thermomechanical technique which hasbeen successfully used to produce razor blade stock from Alloy B: Step1.

Alloy B plate (4 in. X 12 in. X 0.6) was hot rolled at 2,l00 F. toproduce a coiled band approximately 0.008 inch thick and 4 inches wide.

Step 2.

The coil was austenitized at from 2,000 to 2,200 F. for 1 hour, cooledto and held for one hour at l,900 F., cooled to ambient temperature andfurther cooled to and held for 1 hour at l00 F.

Step 3.

The coil was then tempered at 500 F. for 2 hours, cold rolled to athickness of approximately 0.004 inches (50% reduction) and thenretempered at 800 to 1,100 F., preferably l,000 F., for 2 2 hours.

The resulting razor blade stock was found to be harder than allcommercial stainless steel razor blades tested and markedly morecorrosion resistant.

A number of factors render the steels of this invention particularlyattractive for use in automobile bumpers. First, unlike other stainlesssteels of similar strength, these new steels can be heat-treated(Condition I) to possess the high ductility required in the formation ofbumpers. Second, the steels of this invention are inherently corrosionresistant and therefore do not require the platings necessary onconventional bumper steels. Third, because these steels harden locallyon impact, the effect of a collision is spread over the entire bumperand local damage is reduced.

What is claimed is:

1. A heat-treated stainless steel consisting essentially of:

Element Weight Percent Carbon 0. l 2 to 0.20 Nitrogen to 0.07 Chromium12 to lo Molybdenum 0 to 7 Vanadium 0 to 0.6 Cobalt 12 to 16 Nickel 0.5to 1.5 Niobium 0.] to 0.3 Iron Balance said steel having been subjectedto the following sequence of heat treatments:

21. austenitized at a temperature of from l,600 to l,800F. and cooled toambient temperature; and

b. austenitized at a temperature of from 1,950 to 2,300F., cooled to andheld at a lower temperature of from l,800 to 2,000F., and cooled toambient temperature; and

c. tempered at a temperature of from 500 to 2. A steel according toclaim 1 which, after having been subjected to treatment (b) and beforehaving been subjected to treatment (c), has been cooled to and held at atemperature of from -65 F. to -l50 F.

3. A steel according to claim 1 which in step (b) was air cooled or oilquenched to ambient temperature.

4. A steel according to claim 2 which in step (b) was air cooled or oilquenched to ambient temperature.

5. A heat-treated stainless steel consisting essentially of:

said steel having been subjected to the following sequence of heattreatments:

a. austenitized at a temperature of from l,600 to 1,800F. and cooled toambient temperature; and

b. austenitized at a temperature of from l,950 to 2,300F.. cooled to andheld at a lower temperature of from l,800 to 2,000F., and cooled toambient temperature; and

c. tempered at a temperature of from 500 to 6. A steel according toclaim 5 which, after having been subjected to treatment (b) and beforehaving been subjected to treatment (c), has been cooled to and held at atemperature of from 65 to l50 F.

7. A steel according to claim 5 which in step (b) was air cooled or oilquenched to ambient temperature.

8. A steel according to claim 6 which in step (b) was air cooled or oilquenched to ambient temperature.

1. A HEAT-TREATED STAINLESS STEEL CONSISTING ESSENTIALLY OF:
 2. A steelaccording to claim 1 which, after having been subjected to treatment (b)and before having been subjected to treatment (c), has been cooled toand held at a temperature of from -65* F. to -150* F.
 3. A steelaccording to claim 1 which in step (b) was air cooled or oil quenched toambient temperature.
 4. A steel according to claim 2 which in step (b)was air cooled or oil quenched to ambient temperature.
 5. A heat-treatedstainless steel consisting essentially of:
 6. A steel according to claim5 which, after having been subjected to treatment (b) and before havingbeen subjected to treatmenT (c), has been cooled to and held at atemperature of from -65* to -150* F.
 7. A steel according to claim 5which in step (b) was air cooled or oil quenched to ambient temperature.8. A steel according to claim 6 which in step (b) was air cooled or oilquenched to ambient temperature.